The
Repair Guys
Spring Stretching
Question:
In our line of work, we
field questions from
contractors and technicians concerning
repairs, installations,
and general backflow
prevention practices.
We’d like to share some
questions that we
receive as well as our
answers. Everyone
has different opinions
on these subjects and
we would like to hear
yours. Contact us with
questions and ideas via
email at: [email protected] or mail us
at American Backflow
Products Co., PO Box
37025, Tallahassee,
Florida 32315.
Mark Inman and
Doug Taylor
I am working on a 1 ½ inch Wilkins 975
RPZ. I am having an issue with the second
check valve failing the differential test but
holding against backpressure. I have already
replaced the check rubber and seat. Do I need
to replace the spring or can I just stretch it to
make it stronger?
Doug:
Believe it or not, this is a very common question. We will start by taking a closer look at the
check valve assembly of the 975. This particular assembly utilizes a poppet style check
design. A poppet style check consists of a seat,
spring, and poppet. The poppet contains the
rubber disc and has a stem that is the guide.
The lid of the check valve has a recess that
acts as the guide surface. The stem guide is
positioned inside the check lid. You will find
the poppet style check in the Wilkins 950 &
975 series, Conbraco 40-100 & 40-200 series,
and Febco 805Y & 825Y series. Remember to
use caution when you remove the check lid
because the spring is not contained. You made
the correct decision to look at the check rubber
and seats initially because many times this is
where the problem is found. It is normal to
think that if the seat and disc look OK, that the
spring must be the next problem. Be sure not
to overlook the guide surface of the check valve
as a potential problem as well.
Mark:
Let’s concentrate on your question about the
spring first. When a check valve does not seal
tight, many technicians will assume that there
is not enough spring tension so they may try
to add spring tension. When someone says
they want to stretch the spring I always have
to ask do you mean stretch or bend. If you
stretch a spring, it goes back to its original
shape very quickly. This will have little if any
affect on the check valve. If you bend a spring,
you pull it to a point where you permanently
change the shape and size of the spring. This
will have a major effect on the check valve.
You have changed the equipment inside
the backflow and “reengineered” the valve.
Springs are designed and manufactured to
last the life of the check valve and normally
do not need to be replaced. I also believe that
you never-say-never. There are times when
springs can break or weaken and need to be
replaced, but bending a spring in the field
may just cover up the actual problem and not
fix it correctly.
Doug:
So, if it is not the spring, where is the next
place that we want to look? This brings us
to the guide surface for the check poppet.
A guide bore is drilled into the check valve
14 March 2009 | DW&BP
The Repair Guys
cover, which when operating correctly, keeps
the poppet properly aligned so it can form
a uniform seal against the seat. The poppet
stem should slide smoothly in and out of this
guide bore. If there is a build up of debris
or scale inside the guide bore this will cause
the poppet to catch or stick inside the cover
which will slow the travel of the poppet. When
the movement of the poppet is slowed, it may
still close tight, but it can cause low or failing
gauge readings on the differential test. When
you conduct a backpressure test, the check is
forced closed and it passes. Be sure to clean
and inspect the guide bore and poppet. Do not
use any type of lubricant on the poppet stem
or guide bore. Lubricants will also slow down
the travel of the poppet.
Mark:
Many times you will find guide problems on
the second check because the check spring
has a lighter load. One tip that we have found
to help with guide issues on the second check
involves the position of the guide bore. When
the cover is manufactured, the guide bore is
not always drilled exactly center. This will
cause the poppet to bind as it travels through
the guide bore. To compensate for this, simply loosen the check lid about a ¼ turn. This
should be enough to realign the guide bore
so that the check can travel with ease. If this
works, leave the check lid in this position. The
lid is o-ring sealed so loosening it a little bit
will not cause it to leak.
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